In recent years, there has been proposed an illumination apparatus with a visible light communication function to transmit a variety of information in a free space by using illumination light. Such illumination apparatus is disclosed in, e.g., Japanese Patent Application Publication No. 2011-34713. The illumination apparatus described in Japanese Patent Application Publication No. 2011-34713 includes a light emitting portion substrate on which light emitting diodes serving as semiconductor light emitting elements are disposed, a lighting circuit substrate which is connected to the light emitting portion substrate to control lighting of the light emitting diodes, and a visible light communication control substrate which performs a visible light communication control of the light emitting diodes. The visible light communication control substrate is detachably connected between the lighting circuit substrate and the light emitting portion substrate, and thus, in this conventional example, it is possible to generalize the design of the illumination apparatuses with/without the visible light communication function.
In the conventional example, the visible light communication is carried out by modulating an intensity of the illumination light based on an information signal (communication signal) to be transmitted. In other words, in an illumination light communication device on the transmitting side, the communication signal is superimposed on the illumination light by turning on/off an illumination light outputted from a light source unit including the light emitting diodes as a light source. Then, a receiver on the receiving side receives the communication signal by detecting a difference between a light intensity when the pulse of the communication signal is not superimposed and a light intensity when the pulse of the communication signal is superimposed.
When modulating the intensity of the illumination light as described above, in order to make it easier to detect a difference in light intensity, the light intensity is modulated, e.g., at a modulation degree of 50%, i.e., such that 50% of a maximum of the load current becomes a minimum of the load current. In case of modulating the light intensity while maintaining a constant modulation degree as described above, if dimming is controlled to result in a high (bright) light level, it is easy and no problem to detect the difference in the light intensity because the difference between the maximum and the minimum of the load current is large enough. However, if dimming is controlled to result in a low (dark) light level, it becomes difficult to detect the difference in the light intensity since the difference between the maximum and the minimum of the load current decreases, which causes a problem in that it becomes difficult to receive the communication signal and fine communication cannot be maintained.